1. Technical Field
The present invention relates to processing of printing data, scanned in a screen of image points from an image pattern which can include both line work and continuous tone portions and computer stored as pixel data in a raster or rasters corresponding to the scanning screen, making the data suitable for use in electromechanical engraving of printing forms with a screen of printing cells arranged in nested or staggered rows different from the screen arrangement of the pixel data raster(s) derived from the scanned image. More particularly, the invention relates to a gravure data processor by which stored image pixel data comprising tonal density dual-value line work pixels in a higher line-concentration raster, and multiple tone-value picture pixel data in a lower line-concentration raster, are converted into a format of engraving pixel data for realizing a nested- or staggered-row screen of gravure printing cells, image density weighted for high-fidelity reproduction of tone value and line sharpness of the originally scanned image pattern.
2. Description of the Background
Half-tone scanning of a printing subject will produce image information corresponding to a screen of scanned image pattern points which can be digitalized and stored as pixel data. The image density of the pixels (corresponding to the point tone values of the scanned image) and the format in which the pixel data are arranged will depend upon the type of printing process for which the pixel data is to be used. For offset printing, the image pattern pixel data corresponding to the printing screen of dots is mapped in rectangular rasters of pixels representing a relatively high concentration of printing dots, from between 300 to 2,000 dpi (dots/in), for example depending upon the image resolution; for character or line work, the dual-valued pixel data "0" or "1", i.e., no tone value, or full printing tone value should be in a raster of higher pixel concentration than that of the pixel raster for printing the continuous tone (picture) portions of the subject image pattern.
In contrast, in gravure printing, the cells electromechanically engraved into a printing form are arranged in lines of lower density, for example, 150 to 200 lines/in, compared with the dots/in density in offset printing, wherein each cell can be cut in a range of sizes (depths) corresponding to multiple tone-value gradations (256 in the case of 8-bit tone value resolution). If each gravure cell is to correspond to pixel data from the half-tone, digitally stored scan, the gravure pixel data must represent cells arranged in a screen substantially different from the offset printing screen. Thus, whereas pixels comprising image data for offset printing are arranged in a rectangular raster, pixels comprising image data for gravure engraving must be arranged in a format for engraving the cells in nested or staggered rows wherein non-terminal cells are each at the center of a quincunx having rectangular vertices which will not be coincident with the vertices of the offset printing screen.
Therefore, since the printing screens (and hence the respective pixel arrangements in different concentration formats) for offset and for gravure printing do not correspond, image pixel data scanned for offset printing cannot be used unaltered as engraving pixel data for gravure printing.
Accordingly, when electromechanical engraving of a gravure printing form carried out using the subject image pixel data for offset printing is desired, conventionally the offset printing pixel data are read out once from the computer store, and accordingly a film of the subject is produced. The film is then scanned by a gravure engraving system scanner to provide the electronic image for engraving the printing form. This procedure is complicated and time consuming.
Where line work including characters, lines, or other writings, together with a continuous tone picture which can be a color image are to be gravure printed on the same sheet or web, according to the conventional example, it is necessary to make a film composite of combined images, laying out the line work and the picture image in the required positions. Initially in this case, monochromatic, half-tone color separation films are produced for the continuous tone (picture) portion of the subject by a color separation scanner, and a line work film is produced for the corresponding portions of the subject from the line work data by a line work output machine. A single film composite is then made from the color separation films and the line work film.
Engraving data input means in the gravure electromechanical engraving system generally includes an input section for data scanned from line work film, and a separate input section for data scanned from half-tone separations, either of which is accordingly used depending upon the nature of the film composite. The input section receiving data scanned from half-tone color separations has a feature which modifies the screen for each separation in order to prevent moire effects; however, this reduces printing fidelity. Meanwhile, although in the line-work input section the image quality from the line work film is superior to that in the color-separation input section from the half-tone films, if half-tone data is input into the line-work input section, moire effects arise. Consequently, data scanned from a film composite combined from an image comprising both line work and continuous tone (picture) portions has to be input into the half-tone input section, degrading the picture quality of the linework.